A liquid crystal display panel comprises a two-dimensional liquid crystal pixel matrix, and driving devices for the liquid crystal display panel includes a gate driving device and a data driving device. The data driving device sequentially latches the display data as input and converts the same to analogy signals, and scans data lines of the liquid crystal display panel sequentially. The gate driving device includes a plurality of stages of shift registers, each stage of shift register transforms a clock signal as input into ON or OFF signal and outputs the same to a corresponding gate line via a control signal output terminal thereof.
In some special cases, there is a need for rotating a picture displayed on the liquid crystal display panel by 180°. So, the shift registers in the liquid crystal display panel should be able to achieve a bi-directional scanning, that is, the shift registers in the liquid crystal display panel should be bi-directional scanning shift registers.
A typical structure of an existing bi-directional scanning shift register is as shown in FIG. 1. During a non-operational time period of an Nth stage of shift register, that is, during a time period other than a time period in which a gate line corresponding to the Nth stage of shift register is activated and a time period in which a gate line corresponding to a stage of shift register previous to the Nth stage of shift register is activated, the ON signal VON is a high level signal and is input to a gate and a source (or a drain) of a transistor M6 so as ensure that a gate of a transistor M2 and a gate of a transistor M4 receive a high level signal, such that the transistor M2 and the transistor M4 are turned on. Since a source (or a drain) of the transistor M2 and a source (or a drain) of the transistor M4 are connected to the OFF signal VOFF which is a low level signal at this time, the transistor M2 can continuously discharge a node of the Nth stage of shift register which outputs a control signal GOUT(N), and the transistor M4 can continuously discharge a gate of a transistor M1, such that noise in the control signal GOUT(N) of the Nth stage of shift register can be removed, and drift in a threshold of the transistor M1 can be reduced. During a forward scanning, the stage of shift register previous to the Nth stage of shift register refers to an (N−1)th stage of shift register; during a reverse scanning, the stage of shift register previous to the Nth stage of shift register refers to an (N+1)th stage of shift register. In FIG. 1, the bi-directional scanning shift register further includes a clock signal CLK, a forward scan signal Vbuf, a control signal GOUT(N−1) of the (N−1)th stage of shift register, a reverse scan signal Vdis, a control signal GOUT(N+1) of the (N+1)th stage of shift register, a transistor M3, a transistor M5 and a transistor M7.
However, during time periods other than the time period in which the gate line corresponding to the Nth stage of shift register is activated and the time period in which the gate line corresponding to the stage of shift register previous to the Nth stage of shift register is activated, since the gate of the transistor M2, the gate of the transistor M4 and the gate of the transistor M6 in the Nth stage of shift register will be at a high level all the time, the transistor M2, the transistor M4 and the transistor M6 will be in a turn-on state all the time; though the noise produced by the coupling of the clock signal at the control signal output terminal of the shift register is removed, the threshold voltage of the transistors which are in the turn-on state all the time will drift, thus lifespan of the gate driving circuit is shortened.
To sum up, although the shift registers in the conventional gate driving device can remove the noise at the control signal output terminal of the shift register generated by coupling of the clock signal during the non-operational time period, that is, the noise on a gate line to which a shift register is connected can be removed during a time period in which the gate line is not activated, threshold voltages of some transistors in the shift register will drift as they are in the turn-on state all the time during the non-operational time period, and thus the lifespan of the gate driving device is shortened.